I voted for the Vortex of Mortal Magic, since the metaphysics of magic is oftentimes neglected in fantasy worlds. Even my all-time favorite fantasy world, Krynn of the Dragonlance saga, leaves its lunar gods of magic forever shrouded in mystery, even as there is enough Dragonlance background material to make your head asplode. The Vortex and the struggles over its control are key parts of Avlis' contemporary storyline and even now represent one of the most visible aspects of magic in this world.

I voted Neverdead because it's such a simple concept. Simple yet brilliant, it adds a whole new spin to the game. It fits so seamlessly into the standard D&D mythology that I'm amazed it wasn't already introduced in all the hundreds of supplements that have been published throughout the years. Nice!